1. Field of the Invention
The invention relates to welding equipment and in particular to a propulsion unit for driving welding equipment inside pipe sections being welded together to form a pipeline.
2. General Background
In operations such as laying continuous pipeline on land or offshore, a common method of welding the pipe sections together is by flash butt welding. In this type of operation the pipe sections are clamped together end-to-end and heated electrically by electrical contact shoes. As the temperature of each pipe end increases and reaches forging temperature, the pipe ends are forced together to expel molten metal and undesirable oxides and forge the two pipe ends together. The slag or flash, sometimes referred to as upset metal, is then removed by specialized flash removal devices. Another pipe section is then moved into position to be added to the pipeline and the process is repeated. Since equipment such as clamping, welding, flash removal, and measuring devices are normally positioned inside the pipe during these operations, it is necessary to have some means of effectively moving this equipment within the pipe sections to the proper positions for carrying out such operations. Applicant is aware of the following units aimed at driving machinery inside of pipes.
U.S. Pat. No. 3,044,431 entitled "Internal Pipe Clamp" discloses a pivoted link with a pair of traction wheels rotatably mounted at the free end. The link is pivotally attached to the bottom of an upwardly extending arm which is attached at its top to an air operated piston and cylinder. Pressurization of the air cylinder causes movement of the wheels into contact with the pipe while a spring on the arm causes the wheels to move out of contact with the pipe when air pressure is released. The wheels are chain driven by an air motor mounted on the framing ring of the clamp.
U.S. Pat. No. 3,633,813 entitled "Internal Pipe Lineup Clamp" discloses driving wheels rotatably mounted on a frame moved up and down by a piston and cylinder combination to cause the drive wheels to move into and out of contact with the pipe. The wheels are chain driven by an air motor.
U.S. Pat. No. 4,436,974 entitled "Pipe Resistance Butt Welding Apparatus" discloses drive wheels fitted to spring loaded levers and connected through a worm gear and a gimbal gear to a rotation drive.
U.S. Pat. Nos. 2,638,069 entitled "Pipe Aligning Device", 2,594,000 entitled "Electric Line-Up Clamp", and 1,427,231 entitled "Electric Seam Welding Machine" disclose various drive mechanisms.
In welding operations such as that described above, the apparatus inside the pipe may be as much as one hundred (100) feet long and weigh as much as twenty-five (25) tons. This presents the need for a driving unit capable of providing substantial driving wheel contact with the pipe and rotational force to the driving wheel for reliably moving such equipment in the pipe.